"Thunder Afloat" is a very unusual film. Although it appears to be a film about WWI, it really seems to be about WWII. However, the US was officially neutral in 1939 and Congress actually enacted legislation forcing the studios to remain neutral (though this law clearly violated the US Constitution). So, studios were VERY apprehensive to speak out against Hitler and Nazis. But, by 1939 it appeared as if the US might be entering the war eventually and the human rights violations were too great to ignore--plus the war just broke out in Europe. So if you wanted to find an anti-Nazi film in the US in 1939, you'd be VERY hard-pressed to find one aside from the VERY brave "Confessions of a Nazi Spy" (1939). In order to bolster anti-German sentiment, "Thunder Afloat" shows Germans as evil...but this film is set in 1917, not 1939--a clever way to get around the stupid neutrality laws.
The film begins with an old tugboat captain (Wallace Beery) tricking his 'friend' (Chester Morris) into joining the Navy during WWI. However, soon Beery himself would join after his tugboat was sunk by a German sub. Beery is given command of a patrol boat but mucks it up badly. He won't take orders and acts as if he's a one-man Navy--and ends up is serious hot water with his commander--who is, incidentally, Morris!! Is there any redemption or will Beery just be a dope and the US lose eventually the war because of him? Is there any doubt! Again, like so many US military films made about that time, the MAJOR theme is teamwork--most notably the power of the group over the individual. However, the film manages to be both entertaining and fun--with a very rousing finish. So, despite following formula, it does so very well.